PROJECT SUMMARY As nearly half of new HIV infections are thought to be acquired from those unaware of their status, novel strategies are needed to find HIV-infected individuals who are not yet linked to care and others who are at increased risk for acquiring HIV. Current strategies use social network (i.e. contact tracing and analyses of sex/drug use partner data to identify those at risk for infection), spatial (i.e. locate and characterize the social environment of ?hot spots), and phylogenetic (i.e. infer/characterize putative transmission links among HIV- infected persons based on viral sequence similarities) approaches. Combining these three strategies is rare, but could lead to a more comprehensive understanding of HIV transmission and the development of better targeted and multi-level intervention approaches to identify new HIV infections and prevent further transmission to others. Each approach has limitations when used alone, but they can complement and validate one another when used together. Social network data on recent sex/drug use partners provide insight into who is at risk for acquiring HIV based on the current network structure and existing relationships. Phylogenetic links are inferred between HIV-infected persons, typically constructed retrospectively using de-identified samples, and often not validated with data on sex/drug use partners. Without data on sex/drug use partners, two genetically similar viral sequences could be due to direct transmission, indirect transmission via a common source, or a series of intermediaries. Further, because phylogenetic links only include HIV-infected individuals, they provide limited information about who might be at risk for future infection. Thus, combining social network and phylogenetic links could uncover bridges between seemingly distinct networks. Finally, because individual risk behaviors are influenced by the people with whom they interact and the places they go to meet partners and/or engage in risk behaviors, analyzing social network and phylogenetic data together with spatial data can provide social context for these interactions. The specific aims are to (1) Combine social network and spatial data from HIV-infected individuals and their sex/drug use partners to identify novel combinations of network- and spatial/venue-based strategies to find new HIV infections and prevent further transmission; and (2) Overlay putative phylogenetic transmission ties with social network and spatial data and compare HIV intervention strategies identified in Aim1 with those identified with this combined approach. To achieve the Aims, newly diagnosed HIV infections (N=50) from Boston Medical Center (BMC) and their peer-referred drug/sex partners who are perceived to be HIV-negative (N=75) will complete a survey to collect demographic, behavioral, social network, and spatial data (Aim 1). HIV pol sequences from HIV-positive participants and 500 additional consenting BMC HIV patients will be obtained from routine drug resistance testing, used in phylogenetic analyses, and combined with Aim 1 social network and spatial data (Aim 2). These aims will identify priority intervention targets at each level and novel combinations which maximize coverage to those most at risk of HIV, without duplicating efforts.